package/ci: fix code coverage crashes for plugin tests on macOS.

This cost me the whole weekend :(

package/ci/travis.yml

@@ -81,6 +81,14 @@ matrix:
     - LCOV_EXTRA_OPTS="--gcov-tool /usr/bin/gcov-4.7"
   - language: cpp
     os: osx
+    # The new plugin testing workflow is dynamically loading and unloading them
+    # in the tests. When that's combined with coverage and LLVM < 5.0.1, all
+    # such tests crash on exit: https://bugs.llvm.org/show_bug.cgi?id=27224
+    # But only in some cases (for example not at all in tests for Corrade
+    # PluginManager itself). The commit fixing it is from Jan 3 2018 and all
+    # Xcode versions before Xcode 9.3 beta (released on Jan 24) are affected.
+    # https://github.com/llvm-mirror/compiler-rt/commit/860d7953a6f36980f406bfbff76f3c00f15eed57
+    osx_image: xcode9.3beta
     compiler: clang
     env:
     - JOBID=macos-desktop